jpountz commented on a change in pull request #905: LUCENE-8990: Add
estimateDocCount(visitor) method to PointValues
URL: https://github.com/apache/lucene-solr/pull/905#discussion_r331111490
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File path: lucene/core/src/java/org/apache/lucene/index/PointValues.java
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@@ -241,9 +241,28 @@ default void visit(DocIdSetIterator iterator, byte[]
packedValue) throws IOExcep
* than {@link #intersect(IntersectVisitor)}.
* @see DocIdSetIterator#cost */
public long estimateDocCount(IntersectVisitor visitor) {
- return (long) Math.ceil(estimatePointCount(visitor) / ((double) size() /
getDocCount()));
+ long estimatedPointCount = estimatePointCount(visitor);
+ int docCount = getDocCount();
+ double size = size();
+ if (estimatedPointCount >= size) {
+ // math all docs
+ return docCount;
+ } else if (size == docCount || estimatedPointCount == 0L ) {
+ // if the point count estimate is 0 or we have only single values
+ // return this estimate
+ return estimatedPointCount;
+ } else {
+ // in case of multi values estimate the number of docs using the
solution provided in
+ //
https://math.stackexchange.com/questions/1175295/urn-problem-probability-of-drawing-balls-of-k-unique-colors
+ // then approximate the solution for points per doc << size() which
results in the expression
+ // D * (1 - ((N - n) / N)^(N/D))
Review comment:
maybe clarify what are D, N and n?
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